The weighted average formula
A weighted average multiplies each value by its weight, sums those products, and divides by the sum of the weights. If three exam scores are 90, 80, and 70 with weights 50%, 30%, and 20%, the weighted average is (90 x 0.5 + 80 x 0.3 + 70 x 0.2) / 1 = 83.
When all weights are equal, the weighted average reduces to the simple arithmetic mean. The distinction matters only when weights differ.
Weighted average = Sum(value_i x weight_i) / Sum(weight_i)
Each value is multiplied by its weight, the products are summed, and the result is divided by the total weight.